In revision 96fefaa02ae3 the behaviour of Synopsis was changed to break on
":\n". This means that the comment for Synopsis is now incorrect wrt
definition of the string end point (in addition to the returned string not being a
sentence):

// Synopsis returns a cleaned version of the first sentence in s.
// That sentence ends after the first period followed by space and
// not preceded by exactly one uppercase letter. The result string
// has no \n, \r, or \t characters and uses only single spaces between
// words. If s starts with any of the IllegalPrefixes, the result
// is the empty string.